Devices for measuring an angle formed in a workpiece by bending directly during the bending operation in a bending press have already been proposed.
One of these known devices, which is shown in FIG. 1, provides for the use of two or more distance-measuring sensors 100 disposed on each face 102 of the V-shaped groove in the die 104. This known device calculates the angle formed in the workpiece by bending, by means of direct distance measurements between the surface of the piece and the respective wall 102 of the V-shaped groove in the die 104, made with pointed feelers 100 at two points spaced a known distance apart in a direction perpendicular to the bending line. The main disadvantage found with this solution results from the need to miniaturize the device if it is to be used in "real time", that is, immediately after the bend has been made with the sheet metal still in the die, just after the punch has been raised to leave the sheet metal free. In fact, in order to obtain an accurate measurement of the bend angle, it is necessary to measure the distance between the workpiece and the wall of the V-shaped groove with an accuracy of the order of one micron, particularly when the V-shaped groove itself is small. Although the sensors can make measurements with this accuracy, the measurement effected is corrupted decisively by the surface roughness of the piece which is of the same order of magnitude as the precision with which the measurement has to be made.